A flexible printed wiring board is a flexible wiring board with a conductive circuit formed on the surface of a polymer insulating film, and has been frequently used as means to achieve the downsizing and densification of recent electronic equipment. Among others, the flexible printed wiring board provided with metal foils on both sides thereof has been in a highly growing demand.
The aromatic polyimide film is generally used as the polymer insulating film. The flexible printed wiring board provided with metal foils on both sides thereof using such a film has been produced by a method wherein metal foil-polyimide films each with a metal foil laminated on one side thereof are adhered with each other using an adhesive containing an epoxy resin, acrylic resin, or the like, as a major component. For this reason, properties of the printed wiring board such as heat resistance, chemical resistance, fire retardancy, electrical properties, and adhesion are affected by the adhesive used, and such excellent properties distinctive of polyimide were not fully utilized.
On the other hand, a method is proposed wherein, using a thermoplastic polyimide film as an adhesive, metal foil-laminated polyimide films are thermally fused through the thermoplastic polyimide film (e.g., see Patent Documents 1 and 2). Alternatively, a method is known wherein metal layers are formed directly on both sides of a polyimide film by vapor deposition or sputtering (e.g., see Patent Document 3).
However, the flexible printed wiring board that uses a thermoplastic polyimide as an adhesive did not have satisfactory resistance to thermal history because the heat resistance of the thermal plastic polyimide was not always sufficient. Further, due to the high molding temperature for the thermal fusion, there was a problem in that an intricate manufacturing facility was required. As for the manufacturing method employing sputtering, there were problems in that a special apparatus for performing sputtering was required as well as a plating step and warm-heat treatment were sometimes involved, whereby the manufacturing process became complicated.
Consequently, polyamide-imide resin type thermosetting adhesives are considered which combine a heat resistant resin such as a polyamide-imide resin and a thermosetting resin and cure by the reaction of these resins (e.g., see Patent Documents 4 and 5).    Patent Document 1: Japanese Patent Laid-Open No. 03-104185    Patent Document 2: Japanese Patent Laid-Open No. 2004-230670    Patent Document 3: Japanese Patent No. 3447070    Patent Document 4: Japanese Patent Laid-Open No. 11-217503    Patent Document 5: Japanese Patent Laid-Open No. 2004-179237